dsh-credentials-vault: HashiCorp Vault Backend

`dsh-credentials-vault` is a HashiCorp Vault plugin for DeepSeek Harness (DSH), designed to replace local secret storage and address secret security and auditing concerns. The plugin provides machine authentication via the AppRole mechanism, supports secret rotation without restarts and centralized management, and includes auditing capabilities. It is compatible with headless mode, containers, and CI environments, and supports only the KV secrets engine. Installation requires specifying the Vault connection details and AppRole credential references in the configuration file, with all sensitive information injected through environment variables.